Crystalline inverted membranes grown on surfaces by electrospray ion beam deposition in vacuum

A Next-Generation qPlus-Sensor-Based AFM Setup: Resolving Archaeal S-Layer Protein Structures in Air and Liquid

Surface-layer (S-layer) proteins form the outermost envelope in many bacteria and most archaea and arrange in two-dimensional quasicrystalline structures via self-assembly. We investigated S-layer proteins extracted from the archaeon Pyrobaculum aerophilium with a qPlus sensor-based atomic force microscope (AFM) in both liquid and ambient conditions and compared it to transmission electron microscopy (TEM) images under vacuum conditions. For AFM scanning, a next-generation liquid cell and a new protocol for creating long and sharp sapphire tips was introduced. Initial AFM images showed only layers of residual detergent molecules (sodium dodecyl sulfate, SDS), which are used to isolate the S-layer proteins from the cells. SDS was not visible in the TEM images, requiring more thorough sample preparation for AFM measurements. These improvements allowed us to resolve the crystallike structure of the S-layer samples with frequency-modulation AFM in both air and liquid.

Multiplexing of Electrospray Ionization Sources Using Orthogonal Injection into an Electrodynamic Ion Funnel

In this contribution, we report an efficient approach to multiplex electrospray ionization (ESI) sources for applications in analytical and preparative mass spectrometry. This is achieved using up to four orthogonal injection inlets implemented on the opposite sides of an electrodynamic ion funnel interface. We demonstrate that both the total ion current transmitted through the mass spectrometer and the signal-to-noise ratio increase by 3.8-fold using four inlets compared to one inlet. The performance of the new multiplexing approach was examined using different classes of analytes covering a broad range of mass and ionic charge. A deposition rate of >10 μg of mass-selected ions per day may be achieved by using the multiplexed sources coupled to preparative mass spectrometry. The almost proportional increase in the ion current with the number of ESI inlets observed experimentally is confirmed using gas flow and ion trajectory simulations. The simulations demonstrate a pronounced effect of gas dynamics on the ion trajectories in the ion funnel, indicating that the efficiency of multiplexing strongly depends on gas velocity field. The study presented herein opens up exciting opportunities for the development of bright ion sources, which will advance both analytical and preparative mass spectrometry applications.

Material and Charge Transport of Large Organic Salt Clusters and Nanoparticles in Electrospray Ion Beam Deposition

Electrospray ion beam deposition (ES-IBD) or ion soft landing has been demonstrated as a technique suitable for processing nonvolatile molecules in vacuum under perfectly controlled conditions, an approach also desirable for the deposition of nanoparticles. Here, we present results from several approaches to generate, characterize, and deposit nanoparticle ion beams in vacuum for deposition. We focus on cluster ion beams generated by ESI of organic salt solutions. Small cluster ions of the salts appear in the mass spectra as defined peaks. In addition, we find nanoparticle-sized aggregates, appearing as a low intensity background at high m/z-ratio, and show by IBD experiments that these clusters carry the major amount of material in the ion beam. This transition from clusters to nanoparticles, and their successful deposition, shows that ES-IBD can in principle handle ion beams of very heavy and highly charged nanoparticles. In related experiments, however, we found the deposition of nanoparticles from dispersions to be of low reproducibility, due to the lack of control by mass spectrometry.

Principles of Operation of a Rotating Wall Mass Analyzer for Preparative Mass Spectrometry

In this contribution, we describe the principles of operation of a rotating wall mass analyzer (RWMA), a mass-dispersive device for preparative mass spectrometry. Ions of different m/z are spatially separated by RWMA and deposited onto ring-shaped areas of distinct radii on a surface. We use a combination of an analytical equation for predicting the radius of the deposition ring and SIMION simulations to understand how to optimize the experimental conditions for the separation of multicomponent mixtures. The results of these simulations are compared with the experimental data. We introduce a universal mass calibration procedure, based on a series of polyacrylamide ions, which is subsequently used to predict the deposition radii of unknown analytes. The calibration is independent of the polarity, kinetic energy, and charge state of the ion as demonstrated by assigning m/z values of different analytes including multiply charged ubiquitin ions. We demonstrate that mass resolution of the RWMA is affected by the width and kinetic energy distribution of the ion beam. The best mass resolution obtained in this study is m/Δm = ∼20. Preparative mass spectrometry using RWMA provides the advantages of simplicity, compactness, and low fabrication cost, which are particularly promising for the development of miniaturized instrumentation. The results presented in this work can be readily adapted to preparative separation of a variety of charged species of interest to the broad scientific community.

Role of Chemical Dynamics Simulations in Mass Spectrometry Studies of Collision-Induced Dissociation and Collisions of Biological Ions with Organic Surfaces

In this article, a perspective is given of chemical dynamics simulations of collisions of biological ions with surfaces and of collision-induced dissociation (CID) of ions. The simulations provide an atomic-level understanding of the collisions and, overall, are in quite good agreement with experiment. An integral component of ion/surface collisions is energy transfer to the internal degrees of freedom of both the ion and the surface. The simulations reveal how this energy transfer depends on the collision energy, incident angle, biological ion, and surface. With energy transfer to the ion's vibration fragmentation may occur, i.e. surface-induced dissociation (SID), and the simulations discovered a new fragmentation mechanism, called shattering, for which the ion fragments as it collides with the surface. The simulations also provide insight into the atomistic dynamics of soft-landing and reactive-landing of ions on surfaces. The CID simulations compared activation by multiple "soft" collisions, resulting in random excitation, versus high energy single collisions and nonrandom excitation. These two activation methods may result in different fragment ions. Simulations provide fragmentation products in agreement with experiments and, hence, can provide additional information regarding the reaction mechanisms taking place in experiment. Such studies paved the way on using simulations as an independent and predictive tool in increasing fundamental understanding of CID and related processes.

DRILL Interface Makes Ion Soft Landing Broadly Accessible for Energy Science and Applications

Polyoxometalates (POM) have been deposited onto carbon nanotube (CNT) electrodes using benchtop ion soft landing (SL) enabled by a vortex-confined electrohydrodynamic desolvation process. The device is based on the dry ion localization and locomotion (DRILL) mass spectrometry interface of Fedorov and co-workers. By adding electrospray emitters, heating the desolvation gas, and operating at high gas flow rates, it is possible to obtain stable ion currents up to -15 nA that are ideal for deposition. Coupled with ambient ion optics, this interface enables desolvated ions to be delivered to surfaces while excluding solvent and counterions. Electron microscopy of surfaces prepared using the device reveal discrete POM and no aggregation that degrades electrode performance. Characterization of POM-coated CNT electrodes in a supercapacitor showed an energy storage capacity similar to that achieved with SL in vacuum. For solutions that produce primarily a single ion by electrospray ionization, benchtop SL offers a simpler and less costly approach for surface modification with applications in catalysis, energy storage, and beyond.

Two-Dimensional Folding of Polypeptides into Molecular Nanostructures at Surfaces

Herein we report the fabrication of molecular nanostructures on surfaces via two-dimensional (2D) folding of the nine amino acid peptide bradykinin. Soft-landing electrospray ion beam deposition in conjunction with high-resolution imaging by scanning tunneling microscopy is used to fabricate and investigate the molecular nanostructures. Subnanometer resolved images evidence the large conformational freedom of the molecules if thermal motion is inhibited and the formation of stable uniform dimers of only one specific conformation when diffusion can take place. Molecular dynamics modeling supported by density functional theory calculations give atomically precise insight into the induced-fit binding scheme when the folded dimer is formed. In the absence of solvent, we find a hierarchy of binding strength from polar to nonpolar, manifested in an inverted polar-nonpolar segregation which suppresses unspecific interactions at the rim of the nanostructure. The demonstrated 2D-folding scheme resembles many key properties of its native 3D counterpart and shows that functional, molecular nanostructures on surfaces fabricated by folding could be just as versatile and specific.

Tuning the Surface Chirality of Adsorbed Gly-Pro Dipeptide/Cu(110) by Changing Its Chemical Form via Electrospray Deposition

By changing the ultrahigh vacuum (UHV) deposition method, classical sublimation versus electrospray ionization, one can tune the chemistry of a chiral dipeptide molecule (Gly-Pro, GP), when adsorbed on a Cu(110) surface, from anionic to zwitterionic. This chemical shift will influence the adsorption mode of the dipeptide, either in a three-point fashion in the case of anionic GP molecules with a strong interaction among the copper surface, both O atoms of the carboxylate moiety, and the nitrogen atoms, or in the case of zwitterions GP, the adsorption mode relies on the sole interaction of one carboxylate oxygen atom. These different anchoring modes strongly modify the expression of surface 2D chirality and the supramolecular assemblies with two very distinct unit cells.

Dynamics of Protonated Peptide Ion Collisions with Organic Surfaces: Consonance of Simulation and Experiment

In this Perspective, mass spectrometry experiments and chemical dynamics simulations are described that have explored the atomistic dynamics of protonated peptide ions, peptide-H(+), colliding with organic surfaces. These studies have investigated the energy transfer and fragmentation dynamics for peptide-H(+) surface-induced dissociation (SID), peptide-H(+) physisorption on the surface, soft landing (SL), and peptide-H(+) reaction with the surface, reactive landing (RL). SID provides primary structures of biological ions and information regarding their fragmentation pathways and energetics. Two SID mechanisms are found for peptide-H(+) fragmentation. A traditional mechanism in which peptide-H(+) is vibrationally excited by its collision with the surface, rebounds off the surface and then dissociates in accord with the statistical, RRKM unimolecular rate theory. The other, shattering, is a nonstatistical mechanism in which peptide-H(+) fragments as it collides with the surface, dissociating via many pathways and forming many product ions. Shattering is important for collisions with diamond and perfluorinated self-assembled monolayer (F-SAM) surfaces, increasing in importance with the peptide-H(+) collision energy. Chemical dynamics simulations also provide important mechanistic insights on SL and RL of biological ions on surfaces. The simulations indicate that SL occurs via multiple mechanisms consisting of sequences of peptide-H(+) physisorption on and penetration in the surface. SL and RL have a broad range of important applications including preparation of protein or peptide microarrays, development of biocompatible substrates and biosensors, and preparation of novel synthetic materials, including nanomaterials. An important RL mechanism is intact deposition of peptide-H(+) on the surface.

Mass Spectrometry as a Preparative Tool for the Surface Science of Large Molecules

Measuring and understanding the complexity that arises when nanostructures interact with their environment are one of the major current challenges of nanoscale science and technology. High-resolution microscopy methods such as scanning probe microscopy have the capacity to investigate nanoscale systems with ultimate precision, for which, however, atomic scale precise preparation methods of surface science are a necessity. Preparative mass spectrometry (pMS), defined as the controlled deposition of m/z filtered ion beams, with soft ionization sources links the world of large, biological molecules and surface science, enabling atomic scale chemical control of molecular deposition in ultrahigh vacuum (UHV). Here we explore the application of high-resolution scanning probe microscopy and spectroscopy to the characterization of structure and properties of large molecules. We introduce the fundamental principles of the combined experiments electrospray ion beam deposition and scanning tunneling microscopy. Examples for the deposition and investigation of single particles, for layer and film growth, and for the investigation of electronic properties of individual nonvolatile molecules show that state-of-the-art pMS technology provides a platform analog to thermal evaporation in conventional molecular beam epitaxy. Additionally, it offers additional, unique features due to the use of charged polyatomic particles. This new field is an enormous sandbox for novel molecular materials research and demands the development of advanced molecular ion beam technology.

Soft- and reactive landing of ions onto surfaces: Concepts and applications

Soft- and reactive landing of mass-selected ions is gaining attention as a promising approach for the precisely-controlled preparation of materials on surfaces that are not amenable to deposition using conventional methods. A broad range of ionization sources and mass filters are available that make ion soft-landing a versatile tool for surface modification using beams of hyperthermal (<100 eV) ions. The ability to select the mass-to-charge ratio of the ion, its kinetic energy and charge state, along with precise control of the size, shape, and position of the ion beam on the deposition target distinguishes ion soft landing from other surface modification techniques. Soft- and reactive landing have been used to prepare interfaces for practical applications as well as precisely-defined model surfaces for fundamental investigations in chemistry, physics, and materials science. For instance, soft- and reactive landing have been applied to study the surface chemistry of ions isolated in the gas-phase, prepare arrays of proteins for high-throughput biological screening, produce novel carbon-based and polymer materials, enrich the secondary structure of peptides and the chirality of organic molecules, immobilize electrochemically-active proteins and organometallics on electrodes, create thin films of complex molecules, and immobilize catalytically active organometallics as well as ligated metal clusters. In addition, soft landing has enabled investigation of the size-dependent behavior of bare metal clusters in the critical subnanometer size regime where chemical and physical properties do not scale predictably with size. The morphology, aggregation, and immobilization of larger bare metal nanoparticles, which are directly relevant to the design of catalysts as well as improved memory and electronic devices, have also been studied using ion soft landing. This review article begins in section 1 with a brief introduction to the existing applications of ion soft- and reactive landing. Section 2 provides an overview of the ionization sources and mass filters that have been used to date for soft landing of mass-selected ions. A discussion of the competing processes that occur during ion deposition as well as the types of ions and surfaces that have been investigated follows in section 3. Section 4 discusses the physical phenomena that occur during and after ion soft landing, including retention and reduction of ionic charge along with factors that impact the efficiency of ion deposition. The influence of soft landing on the secondary structure and biological activity of complex ions is addressed in section 5. Lastly, an overview of the structure and mobility as well as the catalytic, optical, magnetic, and redox properties of bare ionic clusters and nanoparticles deposited onto surfaces is presented in section 6.

Prediction and clarification of structures of (bio)molecules on surfaces

The design of future materials for biotechnological applications via deposition of molecules on surfaces will require not only exquisite control of the deposition procedure, but of equal importance will be our ability to predict the shapes and stability of individual molecules on various surfaces. Furthermore, one will need to be able to predict the structure patterns generated during the self-organization of whole layers of (bio)molecules on the surface. In this review, we present an overview over the current state of the art regarding the prediction and clarification of structures of biomolecules on surfaces using theoretical and computational methods.

Electrospray soft-landing for the construction of non-covalent molecular nanostructures using charged droplets under ambient conditions

An ambient electrospray soft-landing apparatus was designed to create surface-confined networks on highly oriented pyrolytic graphite through ion/surface interactions.

Active conformation control of unfolded proteins by hyperthermal collision with a metal surface

The physical and chemical properties of macromolecules like proteins are strongly dependent on their conformation. The degrees of freedom of their chemical bonds generate a huge conformational space, of which, however, only a small fraction is accessible in thermal equilibrium. Here we show that soft-landing electrospray ion beam deposition (ES-IBD) of unfolded proteins allows to control their conformation. The dynamics and result of the deposition process can be actively steered by selecting the molecular ion beam's charge state or tuning the incident energy. Using these parameters, protein conformations ranging from fully extended to completely compact can be prepared selectively on a surface, as evidenced on the subnanometer/amino acid resolution level by scanning tunneling microscopy (STM). Supported by molecular dynamics (MD) simulations, our results demonstrate that the final conformation on the surface is reached through a mechanical deformation during the hyperthermal ion surface collision. Our experimental results independently confirm the findings of ion mobility spectrometry (IMS) studies of protein gas phase conformations. Moreover, we establish a new route for the processing of macromolecular materials, with the potential to reach conformations that would be inaccessible otherwise.